Means for examining a substance



Dec. 29, 1959 T. G. CHARLES MEANS FOR EXAMINING A SUBSTANCE Filed Sept.13, 1956 I I l I 20 J T IN VEN TOR.

T/zomas G Charz'es.

United States Patent 2,919,413 MEANS FO A SUBSTANCE Thomas G. Charles,Vasteras, Sweden, assignor to Allmiinna Svenska Elektriska Aktiebolaget,Vasteras, Sweden, 2 Swedish corporation Application September 13,195.6,, Serial No, 609,671 Claims priority, application Sweden July 22,1954 10, Claims. (Cl. 331-65),

This invention is a. continuation-in-part (as to all common subjectmatter) (If the QQPending application Serial No. 521,504, filed July 12,1955, now abandoned, by Mr. Sverre Walter and myself for improvements inmetal. etector or magnetic bore-hole logger, which plication will now bepermitted to lapse without prejudic, in view of its being superseded bythis present application It is know that the change in the,magnification factor of. a resonant circuit au e by a b dy moved i o amagneti or elect ic field fr m the. circui is ependent on the ubs ancein the dy, nd t has been Proposed o tili this fle t in he construc ionof metal dete or r r -hole g r and quiva nt as we as means for ng f c lr or T be u ef l in pract ce. a device of this sort must have a veryhigh sensitivity to ang in the magnifi ation f c r- In a k w device ithhigh ns ivity, th esonan circuit is a parallel-resonant circuit used asgrid circuit n scank n sci l The ankl n oscillator consists of twocascaded resistance-coupled vacuum ub Wi h a f db k ne t n ite he anodef th second tube to the grid oi the first tube. The feedback circuitcontains a large series resistance which together with the resonanceimpedance of the parallel-resonant circuit between the grid and thecathode of the first tube constitutes atvoltage divider determining thefeedback ratio of the oscillator. As the resonance impedance of thecircuit is proportional to the magnification factor of the circuit, thefeedback ratio is dependent on the magnification factor, and if theoscillator works on a linear part of the tube characteristics and nearthe critical feedback ratio, the amplitude of oscillation is verysensitive to changes in the magnification factor. Through a measurementof the changes in the amplitude of oscillation it is in this waypossible to detect a piece of metal fed into the coil of the resonantcircuit, even if the diameter of the piece of metal is less than onethousandth the diameter of the coil.

The known device is, however, very sensitive to changes in the tubecharacteristics, and the sensitivity mentioned above is attainable onlyif the oscillator is provided with means for automatic amplitudestabilization, which, however, should be provided with a certaintime-delay so that transient changes in the amplitude can develop. Theamplitude stabilization has to be very powerful, so that one or moreamplifying stages are needed in this circuit in addition to the twotubes in the oscillator.

It is the object of the present invention to provide a means of the kinddescribed which is simpler in design and has higher s nsi iv ty ha p eously kno n i ce It is another object of the invention to provide ameans of the kind described which has the stability necessary for'itsuse as bore-hole logger without the need for any stabilizing .orcompensating arrangements.

A further object is to provide an oscillator design for a means of thekind described-having excellent stability and sensitivity and stillutilizing one single vacuum tube.

A still further object of the invention is to provide an sc l ates desin fornreens 'e the kind d scribed. in which the s ngl os illa o tube mayhe re ied on even for p ificati n of he osc llat ol age wi nt ny p itmnt the sens i y at he. sc llato car caused y lea s connec ed o said ampifi d v ta e- The i v h c s cha acte zed in th t e sonan circ o theoscillator is a series-resonant circuit and that the iced: back ratio ofthe oscillator is, determined by the ratio betwe he ol ag acr s one racti e eleme in he. resonant circuit and the input voltage to saidcircuit.

i ce he ra i be en he vol g ac o he o l o the capacitor in a seriesresonant circuit and the input voltage to the circuit at resonanceequals the magnif caticn a tor o he circ it, he feedb k at o of he scilator always is proportional to the magnif cation factor in such an ar ag me and the f e bac ra o an he ade larg r th ne The a plifi r cons i ung p t the os lla or herefore ne ne ha any h gh vo tage g and may w rkas a p e p wer p ie t p ob l m of nen-lin ar ies and depen en y n nhechara t istics t us eing mal t is e pecia ly adv a eou to utilize acathode connected vacuum tube as oscillator t r w h t e reso nt c uitThe invention will best be understood from they descrip= ticn he e withrefe enc to th c mpan i d w ng, n hi h igs- 1 and 2 sh w Pref rred cnnso the nv tion- Th u damen a d sign t o to is the same in the two ennb ttnent s, b the ube n i 2 is i zed'bc h as oscillato t be and. a amp fi rtithe or the output v lta e. he arran e n n Fig- 2 is further providedwith a circuit for amplitude stabilization,

In ig. l the nume al 1n desig a es a tube ha in a cathod 1 2 d 12 andPlate 13- The Pla e s an ct t to th p n termina of a a ery 1 and the cah de i n series with wo. at ode res stors 11 and 8 co nected to, the inuterm nal of he battery 16 an to g nd te minal 5- A co 20 ndtwg series cnne capacitors Aland 2.2 a e co nec d be ween th g id .2 ct het be andro d 5, the comrncn ernina o the capac ors 2i nd .22 be n can tes ed tothe junction. po nt of he cathod r s ors" 1.8- n pnt term na 25 is ia aenab i g capacitor 26 canneeted to he cathode 11 not he tube. an issn'nn s d to be onnecte to. a voltme e e or e sensi ver measurem n o i da o oi he a nl e f oscillatio which is a m as re on the subst nceinfluenc n the esnant c r u h capac tor 22 shu t n th ca h d resister 1is ar e cn npered to e can or 21 and its ac nce i lo mpared to h esitance in t esi er 17 a the wo kin fre uency of th vi e tube 19 ogethewith the. bat e y in and t e athode p ance co s ting f esi crlfiin s iew h the p ra el nnection o resis or 1 an capac or 22 he earde a aathode-connec ed amplifier lea ed wi h a eries-r sonan ircui can 'st gQt 991 2!) and the ap citor 1. Because 9. t e teedbac m th essre'nt'circ i tn he rid 2 of he u e the shn n r a e: ent orks e a c l n si ct t e ta t at the ta e g n em he'gri 12 t0 the c es e capa itor 22 isless than one. This is so because the necessary voltage gain is providedby the series-resonant circuit. 7

It is a Well-known fact that the ratio between the voltage across thecoil in a series-resonant circuit and the input voltage to the circuitequals the magnification factor of the circuit at resonance. A seriesresonant circuit thus can provide a voltage gain larger than one, incertain cases several hundred times. The voltage of the coil is howeverin quadrature to the input voltage of feedback voltage. It has beenfound, however, that the device obtains its highest sensitivity when thephase displacement in the capacitor 22 is somewhat incomplete, so thatthe oscillator works at a frequency deviating somewhat from theresonance frequency of the series resonant circuit. The purpose of theresistor 18 is to add to the internal resistance of the tube to providea series resistance suflicient for the phase displacement of the outputvoltage. The resistor 18 will even attenuate the output voltage of theamplifier so that the total gain in the oscillator equals one Withoutany saturation in the amp plifier. The sole purpose of resistor 17 is topass direct current through the tube so that the correct working pointon the tube characteristic is obtained and it may be substituted forinstance by a suitable inductance.

Tests have shown that the arrangement according to Fig. 1 in spite ofits simplicity has an excellent sensitivity to changes in themagnification factor of the resonant circuit arising when a substance tobe tested is moved into the magnetic field of the coil 20 or theelectric field of the capacitor 21. The reason is that the feedbackratio of the oscillator is directly proportional to the magnificationfactor of the circuit, at the same time as the excellent stability inthe cathode-connected amplifier makes it possible to let the oscillatorwork very near to its critical feedback ratio. Even if the arrangementaccording to Fig. 1 has excellent stability, it is often advantageous tostabilize the amplitude of oscillation so that slow changes in thecharacteristics of the oscillator does not disturb its function. In thisway it is among other things possible to avoid adjustments of the devicewhen the measuring member, ie the coil 20 or the capacitor 21, changesits position in relation to the surroundings.

An arrangement satisfying this requirement is shown in Fig. 2, in whichthe numerals from Fig. 1 are used on equivalent elements. The pentodetube together with the cathode resistor 17 and the capacitor 22 Work asa cathode-follower and feeds the series-resonant circuit comprising thecoil and the capacitor 21. The control grid 12 of the tube is connectedto the coil 20 via a coupling capacitor 23, which has for its purpose toseparate the direct potential of the grid from ground. The suppressorgrid 41 is connected to the cathode 11, and the screen grid 42 isconnected through a resistor 43 to the positive terminal of the directcurrent source 16 and through a capacitor 44 to the ground terminal 15.The cathode resistor 18 in Fig. 1 is in Fig. 2 substituted by a plateresistor 19, from which an alternating voltage is taken out via acoupling capacitor 27 to a rectifier of voltagedoubler type, comprisingrectifier elements 31 and 32 together with a capacitor 33 being loadedby a resistor 34. The voltage across the capacitor 33, which isnegative, is added to a positive voltage taken out from a potentiometer36, which in series with a resistor 37 is connected to the battery 16.The voltage difierence is fed to the grid 12 of the vacuum tube via atime delaying network comprising a series resistor 38 and a shuntcapacitor 39. A resistor 40 is connected between the grid 12 and thecapacitor 39 in order that the capacitor shall not short-circuit theresonant circuit. The direct voltage fed to the grid is negative, andits magnitude is dependent on the amplitude of oscillation. An increasein the amplitude of oscillation causes an increased negative gridvoltage, which in turn causes a decrease in the voltage gain in thetube. An increase or decrease in the amplitude of oscillation is thuscounteracted, and through a suitable adjustment of the bias voltage fromthe potentiometer 36, the oscillator may be made to work very near toits sensitivity maximum at all times. The timedelay network 38, 39makes, however, the stabilizing circuit inefilective when transientchanges in the amplitude occurs. When a material to be tested passesthrough the electric or magnetic fields from the resonant circuit, avoltage surge is thus fed to the output terminal via the capacitor 26each time an inhomogeneity having loss properties differing from thoseof the base material passes the measuring device. This voltage surge maybe measured or indicated by an instrument 45 or any other suitabledevice, so that a measurement or indication of the inhomogeneity isobtained.

As the tube is working as a cathode-follower the plate voltage of thetube has very small influence on the amplitude of oscillation in theoscillator, and this influence is reduced in the same degree as theamplification factor ,0. of the tube is increased. Across the plateresistor 19 it is therefore possible to obtain an amplified signal, andif the tube is a pentodc, this amplification may be made very large. Thenecessary degree of amplitude stabilization thus can be obtained withoutthe introduction of any separate amplifying stages and the output signalon the terminal 25 will also be so large that it directly can be fed toa measuring or indicating means, for instance a sensitive sudden changerelay.

The embodiments of the invention shown are at present regarded as themost suitable, but the design of the oscillator may of course be variedwithin the range of the invention. For instance it is possible toutilize a transistor instead of the vacuum-tube shown.

I claim as my invention:

1. Means for examining a substance, comprising an oscillator having aseries resonant tank circuit arranged to produce an electromagneticfield in the substance under test, said oscillator comprising a vacuumtube having a cathode, a control grid and a plate, a resistance meansconnected in series with said tube in such a way that it has alinearizing effect upon the plate-current-gridvoltage characteristic ofsaid tube, an impedance means forming no part of said series resonantcircuit and being connected in the cathode circuit of said tube, saidseries resonant tank circuit consisting of a coil and a capacitor beingseries connected across said impedance means. the junction point betweensaid coil and said capacitor being connected to said control grid.

2. Means according to claim 1, in which said vacuum tube is a pentode.

3. Means according to claim 1, in which said impedance means is acapacitor shunted by a resistor.

4. Means according to claim 1, comprising means for measuring orindicating the amplitude of oscillation in said oscillator, connectedacross at least a part of said resistance means.

5. Means according to claim 1, in which said resistance means isconnected in the cathode circuit of said tube.

6. Means according to claim 1, in which said resistance means isconnected in the plate circuit of said tube.

7. Means according to claim 1, comprising a time-delayedamplitude-stabilizing feedback in said oscillator.

8. Means for examining a substance, comprising a vacuum tube having acathode, a control grid and a plate, a source of direct current having apositive terminal and a negative terminal, a resistance means having atap, connected between said negative terminal and said cathode, a coiland a first capacitor being series connected between said negativeterrninal and said tap on said resistance means, said control grid beingconnected to the junction point between said coil and said firstcapacitor, a second capacitor connected between said negative terminaland said tap, an output terminal connected to said cathode.

9. Means for examining a substance, comprising a vacuum tube having acathode, a control grid and a plate, a source of direct current, aresistance means, an impedance means consisting of a resistor shunted bya capacitor, said tube and said direct current source being connected inseries through said resistance means and said impedance means, saidresistance means being connected to said plate, said impedance meansbeing connected to said cathode, a series resonant circuit comprising acoil and a capacitor shunting said impedance means, means includ- IReferences Cited in the file of this patent ing a coupling capacitorconnecting said control grid to an intermediate point in said seriesresonant circuit, a recti- UNITED STATES PATENTS fying means fed by analternating voltage existing across 2,018,080 Martiensen Oct. 22, 1935said resistance means and having an output, a time delay- 5 2,376,610Millington May 22, 1945 ing means including a series resistor and ashunt capacitor 2,408,029 Bazzoni et a1. Sept. 24, 1946 connecting saidoutput to said control grid. 2,501,834 Wheeler Mar. 28, 1950 10. Meansaccording to claim 9, in which a source of 2,550,607 Shoemaker Apr. 24,1951 bias voltage for said control grid is series connected with2,679,007 Sands May 18, 1954 7 said output from said rectifying means.10 2,772,393 Davis Nov. 27, 1956

